This invention relates generally to footwear representing an improvement on the design described in my U.S. Pat. No. 4,272,899 which is hereby incorporated by reference.
The footwear of the present invention comprises a sole of composite construction, that is, the sole is formed of a first material positioned for attenuating the impact forces applied to the foot and other skeletal structures during standing, walking and running, and a second harder material for firmly supporting the foot.
When walking and running, the lateral (outside) portion of the heel is generally the first part of the foot to strike the ground, with the foot then pivoting on the heel to bring the lateral part of the forefoot into contact with the ground. At this point, the foot is supinated (inclined upwardly from the lateral to the medial side of the foot), but rapidly pronates to a neutral position in which the bottom of the heel and the metatarsal heads of the forefoot are in contact with the ground, and in which the central vertical plane of the heel is ideally generally perpendicular to the ground. During this sequence of movements, various muscles and tendons contract to stabilize the foot in preparation for movement from the neutral position back to a supinated position prior to the propulsive phase of the gait cycle. (The propulsive phase is sometimes referred to as toe-off or push-off.) The arch (i.e., the bone structural alignment) of the foot should be firmly supported when the foot is in the neutral position in order to prevent the ligaments, muscles and tendons from becoming over stressed. During toe-off, it is preferable that the second and third metatarsals be firmly supported, and that the first metatarsal head plantarflex (move downward) relative the second and third metatarsal heads. The toes also should be firmly supported during push-off so that they remain straight and thus stronger due to the pillar effect of the phalanges.
In view of the foregoing, it will be observed that certain parts of the foot are subjected to higher stress during standing, running and walking, and that other parts of the foot require different degrees of support for maximum biomechanical efficiency. Moreover, high impact forces to the foot are transferred to other skeletal structures such as the shins and knees. The present invention takes these considerations into account and provides the appropriate amount of shock attenuation and support to different regions of the foot, thus protecting those parts of the foot which are subjected to high impact forces, and allowing other parts of the foot to function in a way which provides maximum efficiency to prepare the body for stresses placed on it.
Further, the stiffness of the sole, and of the outsole in particular, tends to inhibit flexure of the foot in the area of the metatarsal phalangeal joints. These joints are located between the proximal phalanges (i.e., the rear toe bones) and the metatarsals (i.e., the bones overlying the forward portion of the arch). The stiffness of the sole inhibits dorsiflexion during which the phalanges flex upward relative to the metatarsals. Because the aforementioned stiffness inhibits flexure, the footwear may be uncomfortable, especially during break-in. There is a need, therefore, for footwear which is more flexible, particularly in an area generally corresponding to the metatarsal phalangeal joints of the foot.